Natural killer (NK) cells comprise a subset of lymphocytes involved in protection against microbial pathogens and tumors. Because of their cytotoxic capacity, NK cells are able to directly eliminate abnormal cells. Lytic granules (secretory lysosomes) of NK cells, containing perforin and granzymes, are indispensable for NK-cell cytotoxicity because their release results in the induction of target-cell apoptosis. Defects in lytic granule secretion are associated with serious diseases, such as hemophagocytic lymphohistiocytosis and Chediak-Higashi, Hermansky-Pudlak, or Griscelli syndromes. Knowledge about the proteins involved in regulation of lytic granule release is very limited, and the granule-specific protein machinery involved in NK-cell exocytosis is poorly understood. Chediak-Higashi syndrome (CHS) is a rare lysosomal storage disorder caused by mutations in the lysosomal trafficking regulator gene, LYST. LYST encodes a 429 kDa protein with several distinct domains implicated in various aspects of vesicular trafficking: ARM/HEAT, PH, BEACH and WD-40, but its exact function remains to be elucidated. NK cells in CHS patients have been shown to have abnormal morphology and function. Nevertheless, our understanding of NK cell defects in CHS is limited and the exact role of LYST in cytotoxic lymphocyte biology is unknown. Therefore, this project emphasis has been to understand the role of LYST in regulation of human NK cell cytotoxicity. To address this issue, we disrupted LYST expression in human NK cell lines, and the effects of LYST loss-of-function on lytic granule composition, movement, exocytosis and NK cell cytotoxic potential were investigated. We have successfully generated a human cellular model of CHS that can be used to study consequences of LYST deficiency at a cellular level.